Superconducting and normal-state properties of La1.85Sr0.15Cu1−xGaxBO4

Abstract

We have studied the superconducting and normal-state properties of ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$(${\mathrm{CU}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$)${\mathrm{O}}_4$ (0<x<0.03). $T_c$ decreases essentially linearly with the Ga content and reaches 0 K for only 2.2 at. % of Ga, while the carrier concentration reduces by approximately 20% in the doping range studied. The $T_c$ suppression is analogous to that observed by us in the ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$)${\mathrm{O}}_4$ system. The disappearance of superconductivity is most likely due to the complete filling of the Cu local 3d band and not due to the pair-breaking effect induced by nonmagnetic disorder. The Ga doping induces localization of the Cu 3d holes as evidenced from the susceptibility measurements. The normal-state resistivity exhibits a minimum followed by a logarithmic increase as the temperature is lowered. The temperature of the minimum increases linearly with the Ga content. The logarithmic upturn in the resistivity indicates that neither an activation process nor variable range hopping is responsible for the low-temperature electronic transport. Among two other possible mechanisms, the Kondo effect and effect of localization and correlation, the Kondo effect gives a much better description of the temperature dependence of the resisitivity in the whole temperature range (4–300 K) and for all of the samples.